MORE DEBUG RESULTS.
Extreme frustration countdown is now at T minus 4 seconds 
and counting...  :-}

Strange behavior has returned. 
With scan string -*.jpg +*.Big.JPG -*.GIF the following occurs:

NOTE: The only difference between the following 4 scans is 
the number of URLs listed. In each case, I deleted the 
directory containing the previous scan, and set HTTrack 
to "Download the web site(s)."

Scan #1) With one URL listed for scanning, the scan occurs 
as expected. Debug reports that...
0001.JPG -- (wizard) explicit forbidden link
0001.Big.JPG -- (wizard) explicit authorized link

Scan #2) With 2 URLs listed for scanning, the scan does not 
download any JPG files. Debug reports explicit forbidden link 
for BOTH 0001.JPG and 0001.Big.JPG

Scan #3) With 3 or 4 URLs listed for scanning, the scan 
downloads all JPG files. Debug reports explicit authorized 
link for BOTH 0001.JPG and 0001.Big.JPG.

Scan #4) Going back to 2 URLs... again, scan does not 
download any JPG files! Debug again reports explicit 
forbidden link.

Scan #5) Going back to 1 URL... scan works as expected.

I do not know the difference between the first successful 
scan (wherein I downloaded multiple URLs, getting only 
the .big files) and the current crop of failing scans.